This invention relates generally to vibration based power harvesters. More particularly, the present invention relates to an adjustable frequency vibration power harvester potentially for use in industrial process control and monitoring systems.
Process transmitters, or field devices, are used to monitor process parameters, such as pressure, temperature, flow and level of process fluids used in industrial processes. For example, process transmitters are typically employed in manufacturing and industrial facilities at multiple locations to monitor a variety of process parameters. Additionally, process transmitters are used in isolated field locations such as in cross-country pipelines. Process transmitters are typically integrated within a control system such that the process parameter or process fluid can be manipulated such as with a process control loop.
Process transmitters include sensors that produce an electrical output in response to physical changes in the process parameter. For example, capacitive pressure transducers or piezoresistive pressure transducers produce an electrical output as a function of the pressure of a process fluid. Each process transmitter also includes transmitter electronics for receiving and processing the electrical output of the sensor so that the transmitter and process parameter can be monitored locally or remotely. Locally monitored transmitters include displays, such as LCD screens, that show the electrical output at the site of the process transmitter. Remotely monitored transmitters include electronics that transmit the electrical output over a control loop or network to a central monitoring location such as a control room.
Wireless data transmission networks are rapidly becoming the preferred system for remotely monitored transmitters. Each transmitter, however, must be powered by a long-life battery, as power is not available from the control loop, and power outlets, such as 120 VAC utilities, are typically not located nearby or may not be allowed into hazardous areas where the transmitters are located without incurring great installation expense. In wireless mesh networks, each transmitter must be capable of routing messages for itself as well as other devices in the mesh network. The concept of messages hopping from node to node through the network is beneficial because lower power RF radios can be used, and yet the mesh network can span a significant physical area. Thus, power demands for wireless mesh networks are low and power can be generated from low power energy-scavenging devices. For example, vibration power harvesters that convert mechanical kinetic energy to electric potential energy have been used as power generating means for these control systems. The power produced by these power harvesters, however, is dependent on the available vibration source, which typically leads to a small and erratic power supply. Additionally, as the electronics for control systems and transmitters become more sophisticated, the power demand for these devices also increases. In order to increase the available power supply, it thus becomes necessary to stack several energy harvesting devices, which adds sometimes unjustifiable expense to the process monitoring and control system. Thus, there is a need for a more efficient and less expensive energy harvesting device, particularly for use in industrial process control systems.